England one-day captain Alastair Cook is determined to avoid taking his fine form for granted as he leads the side into their series-decider against Sri Lanka at Old Trafford here Saturday.

Cook made a career-best 119 in a six-wicket defeat at Lord's last weekend and followed that up with an unbeaten 95 off just 75 balls as England thrashed Sri Lanka by 10 wickets at Trent Bridge on Wednesday to leave the five-match series all square at 2-2.

Yet he was left out of England's World Cup squad earlier this year because he supposedly made his runs too slowly for the one-day game.

But since being appointed one-day captain, after Test skipper Andrew Strauss quit the shorter format, Essex opener Cook has led from the front with the bat.

And while his performances against Sri Lanka have stunned some observers, they haven't shocked the 26-year-old left-hander himself.

"I know I can score that quickly," Cook told reporters at Old Trafford here on Friday.

"What I've been working on is the right stuff. It's taken a couple of years at Essex working there and playing one-day cricket away from international cricket to make me expand my game.

"It has had a big effect on my Test career and now I?m starting to prove to myself and everyone else that I do have the tempo at the top of the order."

He added: "It's not about sticking it to the critics, it's about scoring runs at the top of the order and helping England win."

Cook, who scored 766 runs in just seven innings during England's Ashes triumph in Australia, having come close to being dropped during the preceding home series against Pakistan, was asked how long his good form could continue.

"Forever hopefully!," he replied. "If anyone knew why form comes and goes, they would be a very rich man.

"You can't always be at your best. Look at me last summer. It wasn't working as well as it is now. And that's what form is," explained Cook.

"I'll keep doing the same stuff. I've not changed the way I practise. I keep striving to get better. The moment you step back and start enjoying the form, you might take your foot off the gas and your game may drop that one percent."

Sri Lanka hammered England by 10 wickets in a World Cup quarter-final in Colombo in March before eventually losing in the final to India, who arrive in England next week for their tour, in Mumbai.

Cook added the one-day series against Sri Lanka and India would give England a good idea of their standing in the 50-over game.

"My first two series (as one-day captain) are against India and Sri Lanka, the two World Cup finalists, and everyone said what strong sides they will be. But we want to test ourselves against the best."

Meanwhile Cook said he'd have no problems giving up leadership responsibilities when Strauss resumed the captaincy for the upcoming four-Test series against India, which starts at Lord's later this month.

"Not at all. I can concentrate hard on the captaincy and then go back into the ranks. It takes a lot out of you and having shorter periods (as captain) gives you the chance to refresh nicely. I'm sure Straussy is raring to go."